1. Field of the Invention
The field of the invention relates generally to interbody spinal fusion implants configured to correct an angular relationship between vertebrae.
2. Description of the Related Art
Implant systems utilizing specially designed spinal instrumentation are often used in surgical procedures concerning spinal conditions. Such implants are used to correct degenerative conditions and facilitate spinal fusion while stabilizing and strengthening the spine. Spinal fusion is the joining of adjacent vertebrae through shared bone. Degenerative conditions which often require spinal fusion surgery include spondylolisthesis, chronic degenerative disc disease, traumatic fractures, and other forms of spinal instability such as scoliosis.
Degenerative conditions may lead to the spinal cord or nerve roots being squeezed as to cause difficulty with back pain, numbness or weakness in one or all extremities, difficulty with instability and gait, as well as clumsiness and proprioception. In extreme cases, certain degenerative conditions can also lead to losing control over the bladder or bowels.
Implant systems used in spinal fusion surgery can be summarized into the following groups: rods; pedicle screws; hooks; plates; and interbody cages. Rods are used in conjunction with hooks and screws to immobilize involved spinal levels and to contour the spine to correct alignment. Screws are implanted into the pedicles of the spinal vertebrae to provide strong anchorage points for rods, which can then be contoured to correct deformities to facilitate spinal fusion. Hooks are used with rods and other implants to anchor them to the vertebrae. Plates, often used in the cervical spine, are designed to conform to the contour of the spine, and are held in place by screws set into adjacent vertebrae. Interbody cages are small, hollow devices packed with bone graft or other osteogenic fusion graft material (i.e., biologics) used to restore lost disc height between two vertebrae.
A problem that is encountered when using interbody cages in spinal fusion surgery is that current devices are bulky, difficult to introduce through an incision into the disc and cannot be expanded, anchored or filled with biologics in situ. Moreover, current devices are designed by engineers on a naked spine, which necessarily lacks an appreciation for the true conditions of spinal fusion surgery. For example, nerve compression, root injury, dural tears and spinal fluid leaks are frequently produced by oversized interbody cages that are significantly larger in size than the entry point allowed by the axilla of the nerve root, which may result in compression of the dorsal root ganglion.
Oversized interbody cages are often needed to accommodate a large disc space, but current devices are inserted via a small entry point, which is bordered by the spinal dura (medially), nerve root and dorsal root ganglion (cranially), pedicle (caudally), and in case of posterior lumbar interbody fusion, the residual lamina (laterally), which is usually removed when transforamenal interbody fusion is done. In some cases this can result in significant post-operative pain and/or neurologic deficit. Another problem with current devices is that they do not fully permit biologics to be injected into interbody cages in situ. Current devices also have problems with revision, adjustment and repositioning when placed in the spine, and cannot be coupled with another interbody cage in situ. Moreover, when using current devices, biologics must first be inserted into the space between the vertebral bodies, and then again inside the interbody cage after said cage is inserted into the spine. Thus, there is a need in the art for an interbody cage that has the capability to introduce biologics into an interbody cage while it is in final position in situ.
While current devices exist, in part, of other expandable cages there is still a need in the art for an expandable interbody cage that is equally concerned with both the safety and well-being of the patient and the utility of the invention. For example, some current expandable devices, when fully expanded, the devices change in length, causing screws to protrude from the interbody cage. This is a safety concern to the patient, particularly if the protruding screws are directed toward the spinal cord. Should the interbody cage then move or dislodge, it will cause a direct puncture into the spinal cord.